Pull static assets from a configurable CDN

Added some logic in our grunt file that allows us to specify a CDN.

Note that the default configured CDN is pulling assets from production. If you want to test this, you could do the following:
 * Add a `static.oae.com` in your /etc/hosts file
 * Replace the cdn url with `static.oae.com`
Needs to be added to `package.json`